Proton conducting electrolyte membrane for use in high temperatures and the use thereof in fuel cells

ABSTRACT

The present invention concerns a proton-conducting electrolyte membrane obtainable by a method comprising the following steps: A) expanding a polymer film with a liquid that contains a vinyl-containing phosphonic acid, and B) polymerisation of the vinyl-containing phosphonic acid present in the liquid introduced in step A). An inventive membrane, thanks to its exceptional chemical and thermal properties, is very versatile in its use and is particularly suitable as a polymer-electrolyte-membrane (PEM) in so-called PEM fuel cells.

The present invention concerns a proton-conducting electrolyte membranefor high temperature applications with a polyvinyl phosphonic acid base,which thanks to its exceptional chemical and thermal properties is veryversatile in its use and is particularly suitable as apolymer-electrolyte-membrane (PEM) in so-called PEM fuel cells.

A fuel cell normally contains an electrolyte and two electrodesseparated by the electrolyte. In the case of a fuel cell one of the twoelectrodes is supplied with a fuel, such as hydrogen gas or amethanol-water mixture, and the other electrode is supplied with anoxidizing agent, such as oxygen gas or air, and in this way chemicalenergy from the fuel oxidation is converted directly into electricalenergy. During the oxidation reaction protons and electrons are formed.

The electrolyte is permeable to hydrogen ions, i.e. protons, but not toreactive fuel such as the hydrogen gas or methanol and the oxygen.

A fuel cell generally has several individual cells, so-called MEAs(Membrane-electrode assemblies), which each contain an electrolyte andtwo electrodes separated by the electrolyte.

As the electrolyte for the fuel cell solid matter such as polymerelectrolyte membranes or liquids such as phosphoric acid are used.Recently the use of polymer electrolyte membranes as the electrolyte forfuel cells has been attracting attention. Basically, two differentcategories of polymer membranes can be identified.

The first category includes cation exchanger membranes comprising apolymer matrix, which contains covalently bonded acid groups, preferablysulphonic acid groups. The sulphonic acid group changes into an aniongiving of a hydrogen ion and therefore conducts protons. Here, themobility of the proton and thus the proton conductivity is directlylinked to the water content. Because of the very good miscibility ofmethanol and water such cation exchanger membranes have a high methanolpermeability and are therefore unsuitable for applications in a directmethanol fuel cell. If the membrane dries out, i.e. as a result of hightemperature, then the conductivity of the membrane and accordingly theperformance of the fuel cell drop dramatically. The operatingtemperatures of fuel cells containing such cation exchanger membranesare thus limited to the boiling temperature of the water. Humidificationof the fuel presents a major technical challenge to the use of polymerelectrolyte membrane fuel cells (PEM-FC), in which conventional,sulphonated membranes such as Nafion are used.

Thus, for example, perfluorosulphonic acid polymers are used for polymerelectrolyte membranes. The perfluorosulphonic acid polymer (such asNafion) generally has a perfluoro-hydrocarbon matrix, such as acopolymer of tetrafluoroethylene and trifluorovinyl, and a side chainbonded onto this with a sulphonic acid group, such as a side chain witha sulphonic acid group bonded onto a perfluoro-alkylene group.

Cation exchanger membranes preferably involve organic polymers withcovalently bonded acid groups, in particular sulphonic acid. Polymersulphonation methods are described in F. Kucera et. al. PolymerEngineering and Science 1988, Vol. 38, No 5, 783-792.

In the following the most important types of cation exchanger membranesare listed which have enjoyed commercial success when used in fuelcells.

The most important exponent is the perfluorosulphonic acid polymerNafion® (U.S. Pat. No. 3,692,569). As described in U.S. Pat. No.4,453,991 this polymer can be placed in solution and then used as alonomer. Cation exchanger membranes are also obtained by filling aporous base material with such a lonomer. Here, for the base materialexpanded Teflon is preferred (U.S. Pat. No. 5,635,041).

A further perfluorinated cation exchanger membrane can be produced asdefined in U.S. Pat. No. 5,422,411 by copolymerisation oftrifluorostyrene and sulphonyl modified trifluorostyrene. Compositemembranes comprising a porous base material, in particular expandedTeflon, filled with lonomers comprising such sulphonyl-modifiedtrifluorostyrene copolymers are described in U.S. Pat. No. 5,834,523.

U.S. Pat. No. 6,110,616 describes copolymers of butadiene and styreneand their subsequent sulphonation for the production of cation exchangermembranes.

A further class of part-fluorinated cation exchanger membranes can beproduced by laser grafting and subsequent sulphonation. Here, asdescribed in EP667983 or DE19844645, a grafting reaction is carried outwith a previously irradiated polymer film, preferably with styrene. In asubsequent sulphonation reaction the sulphonation of the side chainsthen takes place. Simultaneously with the grafting a cross-linking canalso be performed thereby modifying the mechanical properties. Apartfrom the above membranes a further class of non-fluorinated membraneshas been developed by sulphonation of high-temperature stablethermoplastics. Thus membranes in sulphonated polyether ketones(DE4219077, EP96/01177), sulphonated polysulphone (J. Membr. Sci. 83(1993) p.211) or sulphonated polyphenylsulphide (DE19527435) are known.

Lonomers produced from sulphonated polyether ketones are described in WO00/15691.

Furthermore, acid-base blend membranes are known which, as described inDE19817374 or WO 01/18894, are produced by mixing sulphonated polymersand basic polymers.

To further improve the membrane properties a cation exchanger membraneknown from the state of the art can be mixed with a high-temperaturestable polymer. The production and properties of cation exchangermembranes comprising blends of sulphonated PEK and a) polysulphones(DE4422158), b) aromatic polyamides (42445264) or c) polybenzimidazole(DE19851498) are described.

The disadvantage of these cation exchanger membranes is the fact thatthe membrane must be humidified, the operating temperature is limited to100° C., and the membranes have a high permeability to methane. Thecause of these disadvantages is the conductivity mechanism of themembrane, in which the transport of the protons is linked to thetransport of the water molecule. This is referred to as the “vehiclemechanism” (K.-D. Kreuer, Chem. Mater. 1996, 8, 610-641).

In the second category polymer electrolyte membranes have been developedwith complexes of basic polymers and strong acids. Thus WO96/13872 andthe corresponding U.S. Pat. No. 5,525,436 describe a method forproducing a proton-conducting polymer electrolyte membrane, in which abasic polymer, such as polybenzimidazole, is treated with a strong acidsuch as phosphoric acid, sulphuric acid, and so on.

In J. Electrochem. Soc., Vol. 142, No. 7, 1995, pp. L121-L123 the dopingof a polybenzimidazole in phosphoric acid is described.

With the basic polymer membranes known from the state of the art, themineral acid used—to achieve the necessary proton conductivity—(usuallyconcentrated phosphoric acid) is either used after forming oralternatively the basic polymer membrane is produced directly frompolyphosphoric acid as in German patent application Nos. 10117686.4,10144815.5 and 10117687.2. The polymer serves here as a vehicle for theelectrolyte comprising the highly concentrated phosphoric acid orpolyphosphoric acid. Here, the polymer membrane performs other essentialfunctions. In particular, it must have high mechanical stability and actas a separator for the two fuels mentioned in the introduction.

An important advantage of such a phosphoric acid or polyphosphoric aciddoped membrane is the fact that a fuel cell, in which such a polymerelectrolyte membrane is used, can be operated at temperatures in excessof 100° C. without the humidification of the fuels that would otherwisebe necessary. This is based on the property of the phosphoric acid beingable to transport the protons without additional water by means of theso-called Grotthus mechanism (K.-D. Kreuer, Chem. Mater. 1996, 8,610-641).

The possibility of operating at temperatures in excess of 100° C. offersfurther advantages for the fuel cell system. Firstly, the sensitivity ofthe Pt catalyst to gas impurities, CO in particular, is significantlyreduced. CO is produced as a by-product in the reforming of thehydrogen-rich gas from carbon-containing compounds, such as natural gas,methanol or benzene, or also as an intermediate product in the directoxidation of methanol. Typically the CO content of the fuel must be lessthan 100 ppm at temperatures of <100° C. At temperatures in the range150-200° C., however, even 10,000 ppm CO or more can be tolerated (N. J.Bjerrum et. al. Journal of Applied Electrochemistry, 2001,31, 773-779).This leads to important simplifications in the preceding reformingprocess and thus to cost reductions for the fuel cell system as a whole.

A major advantage of fuel cells is that fact that during theelectrochemical reaction the energy of the fuel is converted directlyinto electrical energy and heat. In the process, water results as areaction product on the cathode. A by-product of the electrochemicalreaction is therefore heat. For applications in which only power is usedto drive electric motors, e.g. in automobile applications, or for theversatile use of battery systems, the heat must be dissipated in orderto avoid overheating of the system. Additional energy-consuming devicesare then needed for cooling, which further reduce the overall efficiencyof the fuel cell. For stationary applications such as the centralised ordecentralised generation of power and heat, the heat can be usedefficiently by means of available technologies such as heat exchangers.In order to increase efficiency, high temperatures are sought here. Ifthe operating temperature is in excess of 100° C. and if the differencein temperature between the ambient temperature and the operatingtemperature is great, then it is possible to cool the fuel cell systemmore efficiently or to use small cooling surfaces and to dispense withadditional devices compared with fuel cells which, because of themembrane humidification, have to be operated at below 100° C.

Apart from these advantages such a fuel cell system has a decisivedisadvantage. And this is that phosphoric acid or polyphosphoric acid ispresent as the electrolyte and is not permanently bonded to the basicpolymer by ionic interaction and can be washed away by water. Asdescribed above, water is formed at the cathode during theelectrochemical reaction. If the operating temperature is in excess of100° C., then the majority of the water is drawn off as vapour via thegas diffusion electrode and the acid loss is very low. If the operatingtemperature drops below 100° C. however, for example when the cell isstarted up or shut down or operated at part load if a high power yieldis sought, then the water formed condenses and can lead to more intensewashing away of the electrolyte, highly concentrated phosphoric acid orpolyphosphoric acid. In the method of operation of the fuel celldescribed above this can lead to a constant loss of conductivity andcell performance, which can lower the lifetime of the fuel cell.

Furthermore, the known membranes doped with phosphoric acid cannot beused in so-called direct methanol fuel cells (DMFC). Such cells are,however, of particular interest since a methanol-water mixture is usedas the fuel. If a known membrane with a phosphoric acid base is used,then the fuel cell fails after a relatively short time.

The object of the present invention is therefore to provide a novelpolymer electrolyte membrane in which the washing away of theelectrolyte is prevented. In particular, therefore, it must be possibleto expand the operating temperature range to between <0° C. and 200° C.without the system requiring humidification. A fuel cell containing aninventive polymer electrolyte membrane should be suitable for purehydrogen and for numerous carbon-containing fuels such as natural gas,benzene, methanol and biomass. In this case the membrane should allowthe greatest possible activity of the fuels. The methanol oxidationshould, in particular, be especially high compared with known membranes.

In addition, an inventive membrane should be economical and simple toproduce. A further object of the present invention was to providepolymer electrolyte membranes which have a high conductivity, inparticular a high conductivity over a wide temperature range. In thiscase the conductivity, in particular at high temperatures, should beachieved without additional humidification.

In addition, a polymer electrolyte membrane should be provided which hashigh mechanical stability, e.g. a high modulus of elasticity, a highultimate tensile strength, low creep and high fracture toughness.

Furthermore, another object of the present invention was to offer amembrane that also has a low permeability during operation to thevarious fuels, such as hydrogen or methanol, and the membrane shouldalso have low oxygen permeability.

These objects are achieved by the production of a liquid that contains avinyl-containing phosphonic acid and a method for producing a polymerelectrolyte membrane through expansion of a polymer film in this liquidand the subsequent polymerisation into a polyvinyl phosphonic acid.Because of the high concentration of vinyl phosphonic acid polymer, itshigh chain flexibility and the high acid strength of thepolyvinylphosphonic acid, the conductivity is based on the Grotthusmechanism and the system therefore requires no additionalhumidification. The polyvinylphosphonic acid, which can also becross-linked through reactive groups, forms an inter-penetrating networkwith the high-temperature stable polymers. Therefore the washing away ofthe electrolyte by the product water formed or in the case of a DMFC bythe aqueous fuel is significantly reduced. An inventive polymerelectrolyte membrane has a very low methanol permeability and isparticularly suited to use in a DMFC. Thus long-lasting operation of afuel cell with a number of fuels such as hydrogen, natural gas, benzene,methanol or biomass is possible. In this case the membranes allow aparticularly high activity of these fuels. Because of the hightemperatures, the methanol oxidation can in this case take place withhigh activity. In a particular embodiment these membranes are suited tothe operation of a so-called moisture-forming DMFC, in particular attemperatures in the range 100 to 200° C.

Thanks to the possibility of operating at temperatures in excess of 100°C., the sensitivity of the Pt catalyst to gas impurities, CO inparticular, drops significantly. CO is produced as a by-product in thereforming of the hydrogen-rich gas from carbon-containing compounds,such as natural gas, methanol or benzene, or as an intermediate productin the direct oxidation of methanol. Typically the CO content of thefuel at temperatures in excess of 120° C. can be more than 5,000 ppm,without the catalytic effect of the Pt catalyst being dramaticallyreduced. At temperatures in the range 150-200°, however, even 10,000 ppmCO or more can be tolerated (N. J. Bjerrum et. al. Journal of AppliedElectrochemistry, 2001, 31, 773-779). This leads to importantsimplifications in the preceding reforming process and thus to costreductions for the fuel cell system as a whole.

An inventive membrane has a high conductivity over a wide temperaturerange, and this can also be achieved without additional humidification.Furthermore, a fuel cell which is fitted with an inventive membrane, canalso be operated at low temperatures of, for example, 80° C. without thelifetime of the fuel cell being severely reduced.

In addition, membranes of the present invention have a high mechanicalstability, in particular a high modulus of elasticity, a high ultimatetensile strength, low creep and high fracture toughness. Furthermorethese membranes demonstrate a surprisingly long lifetime.

The object of the present invention is therefore a stableproton-conducting electrolyte membrane obtainable by a method comprisingthe following steps:

-   -   A) expanding a polymer film with a liquid that contains a        vinyl-containing phosphonic acid, and    -   B) polymerisation of the vinyl-containing phosphonic acid        present in the liquid introduced in step A).

The polymer film used in step A) is a film that has an expansion of atleast 3% in the vinyl phosphonic acid containing liquid. Expansion meansan increase in the weight of the film of at least 3%. The expansion ispreferably at least 5%, and particularly preferred at least 10%.

The expansion Q is determined gravimetrically from the mass of the filmprior to expansion m_(o) and the mass of the film after polymerisationaccording to step B), m₂.Q=(m ₂ −m ₀)/m ₀×100

The expansion preferably takes place at a temperature in excess of 0°C., in particular at between ambient temperature (20° C.) and 180° C. ina vinyl phosphonic acid containing liquid that contains at least 5% byweight vinyl phosphonic acid. The expansion can also be performed athigh pressure. In this case the limits are determined by commercialconsiderations and technical possibilities.

The polymer film used for expansion generally has a thickness in therange 5 to 3,000 μm, preferably 10 to 1,500 μm and particularlypreferred [sic-translator]. The production of such films from polymersis generally known, and these are to some extent available commercially.The term polymer film means that the film to be expanded comprisespolymers, and these films can contain other common additives.

The preferred polymers include, inter alia polyolefins, such aspoly(chloroprene), polyacetylene, polyphenylene, poly(p-xylylene),polyarylmethylene, polystyrene, polymethylstyrene, polyvinylalcohol,polyvinylacetate, polyvinylether, polyvinylamine,poly(N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole,polyvinylpyrrolidone, polyvinylpyridine, polyvinylchloride,polyvinylidenechloride, polytetrafluorethylene, polyhexafluorpropylene,copolymers of PTFE and hexafluoropropylene, withperfluoropropylvinylether, with trifluoronitrosomethane, withcarbalkoxy-perfluoralkoxyvinylether, polychlorotrifluorethylene,polyvinylfluoride, polyvinylidenefluoride, polyacrolein, polyacrylamide,polyacryinitrile, polycyanacrylate, polymethacrylimide, cycloolefiniccopolymers, in particular of norbornen; polymers mit C—O-bonds in themain chain, such as polyacetal, polyoxymethylene, polyether,polypropyleneoxide, polyepichlorohydrine, polytetrahydrofurane,polyphenyleneoxide, polyetherketone, polyester, in particularpolyhydroxyacetic acid,polyethyleneterephthalate,—polybutyleneterephthalate,polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolactone,polycaprolactone, polymalonic acid, polycarbonate; polymer C—S-bonds inthe main chain, such as polysulphide ether, polyphenylene sulphide,polyethersulphone; polymer C—N-bonds in the main chain, such aspolyimine, polyisocyanide, polyetherimine, polyetherimide, polyaniline,polyaramide, polyamide, polyhydrazide, polyurethane, polyimide,polyazole, polyazoletherketone, polyazine; liquid crystalline polymers,in particular Vectra and inorganic polymers such as polysilane,polycarbosilane, polysiloxane, polysilicic acid, polysilicate, silicone,polyphosphazene and polythiazyl.

According to a particular aspect of the present inventionhigh-temperature stable polymers are used which contain at least onenitrogen, oxygen and/or sulphur atom in one or a number of recurringunits.

A polymer with high-temperature stability—for the purposes of thepresent invention—is a polymer where it can be used long-lastingly as apolymer electrolyte in a fuel cell at temperatures in excess of 120° C.Long-lasting means that an inventive membrane can be used for at least100 hours, preferably at least 500 hours at a minimum of 120° C.,preferably 160° C., without the performance, which can be measuredaccording to the method described in WO 01/18894 A2, dropping by morethan 50% in relation to the starting performance.

The polymers used in step A) are preferably polymers that have a glasstransition temperature or Vicat softening temperature VST/A/50 of atleast 100° C., preferably of at least 150° C. with a quite particularpreference for at least 180° C.

Particular preference is for polymers that contain at least one nitrogenatom in a recurring unit. Special preference is for polymers thatcontain at least one aromatic ring with at least one nitrogen heteroatomper recurring unit. Within this group, polyazole-based polymers areparticularly preferred. These basic polyazole polymers contain at leastone aromatic ring with at least one nitrogen heteroatom per recurringunit.

The aromatic ring is preferably a five- or six-link ring with one tothree nitrogen atoms, which can be annellated with another ring, inparticular another aromatic ring.

Polyazole-based polymers contain recurring azole units of generalformula (I) and/or (II) and/or (III) and/or (IV) and/or (V) and/or (VI)and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or (XI) and/or(XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI) and/or (XVI)and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI)and/or (XXII)

in which

-   -   Ar are the same or different and denote a tetravalent aromatic        or heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar¹ are the same or different and denote a bivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar² are the same or different and denote a bivalent or trivalent        aromatic or heteroaromatic group, which can be mono- or        multi-cyclic;    -   Ar³ are the same or different and denote a trivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁴ are the same or different and denote a trivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁵ are the same or different and denote a tetravalent aromatic        or heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁶ are the same or different and denote a bivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁷ are the same or different and denote a bivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁸ are the same or different and denote a trivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   Ar⁹ are the same or different and denote a bivalent or trivalent        or tetravalent aromatic or heteroaromatic group, which can be        mono- or multi-cyclic;    -   Ar¹⁰ are the same or different and denote a bivalent or        trivalent aromatic or heteroaromatic group, which can be mono-        or multi-cyclic;    -   Ar¹¹ are the same or different and denote a bivalent aromatic or        heteroaromatic group, which can be mono- or multi-cyclic;    -   X is the same or different and denotes oxygen, sulphur or an        amino group, that carries one hydrogen atom, a group containing        between: 1 and 20 carbon atoms, preferably a branched or        unbranched alkyl or alkoxy group, or an aryl group as an        additional radical;    -   R is the same or different and denotes hydrogen, an alkyl group        and an aromatic group and    -   n, m is a whole number greater than or equal to 10, preferably        greater than or equal to 100.

Inventively preferred aromatic or heteroaromatic groups can be derivedfrom benzene, naphthalene, biphenyl, diphenylether, diphenylmethane,diphenyldimethylmethane, bisphenol, diphenylsulphone, thiophene, furane,pyrrol, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole,1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole,1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole,1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo[b]thiophene,benzo[b]furane, indole, benzo[c]thiophene, benzo[c]furane, isoindole,benzoxazole, benzothiazole, benzimidazole, benzisoxazole,benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole,dibenzofurane, dibenzothiophene, carbazole, pyridine, bipyridine,pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine,1,2,4-triazine, 1,2,4,5-triazine, tetrazine, chinoline, isochinoline,chinoxaline, chinazoline, cinnoline, 1,8-naphthyridine,1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine,pyridopyrimidine, purine, pteridine or chinolizine, 4H-chinolizine,diphenylether, anthracene, benzopyrrol, benzooxathiadiazole,benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine,benzopyrimidine, benzotriazine, indolizine, pyridopyridine,imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine,benzochinoline, phenoxazine, phenothiazine, acridizine, benzopteridine,phenanthroline and phenanthrene, which if necessary can also besubstituted.

In this case the substitution pattern of Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹,Ar¹⁰, Ar¹¹ is arbitrary, in the case of phenylene for example Ar¹, Ar⁴,Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can be ortho-, meta- and para-phenylene.Particularly preferred groups can be derived from benzene andbiphenylene, which if necessary can also be substituted.

Preferred alkyl groups are short-chained alkyl groups with between 1 and4 carbon atoms, such as methyl-, ethyl-, n- or i-propyl- andt-butyl-groups.

Preferred aromatic groups are phenyl- or naphthyl-groups. The alkylgroups and the aromatic groups can be substituted.

Preferred substitutes are halogen atoms such as fluorine, amino groups,hydroxy groups or short-chain alkyl groups such as methyl- orethyl-groups.

Preference is for polyazoles with recurring units of formula (I) inwhich the X radicals within a recurring unit are the same.

The polyazoles can basically also have differing recurring units, whichfor example differ by their X radicals. Preferably, however, there areonly identical X radicals in a recurring unit.

In a further embodiment of the present invention the polymer containingrecurring azole units is a copolymer or a blend that contains at leasttwo units of formula (I) to (XXII), which differ from each other. Thepolymers can be present as block copolymers (diblock, triblock),statistical copolymers, periodic copolymers and/or alternating polymers.

The number of recurring azole units in the polymers is preferably awhole number greater than or equal to 10. Particularly preferredpolymers contain at least 100 recurring azole units.

Within the context of the present invention polymers containingrecurring benzimidazole units are preferred. Some examples of extremelyadvantageous polymers containing recurring benzimidazole units areprovided by the following formulas:

in which n and m is a whole number greater than or equal to 10,preferably greater than or equal to 100.

The polyazoles used in step A), in particular the polybenzimidazole,however, are characterised by a high molecular weight. Measured as theintrinsic viscosity this is preferably at least 0.2 dl/g, in particular0.8 to 10 dl/g, with particular preference for between 1 and 5 dl/g.

Further preferred polyazole polymers are polyimidazole,polybenzthiazole, polybenzoxazole, polytriazole, polyoxadiazole,polythiadiazole, polypyrazole, polyquinoxalines, poly(pyridine),poly(pyrimidine), and poly(tetrazapyrene).

Particular preference is for Celazole made by Celanese, in particularone in which the polymer processed by sieving as described in Germanpatent application No. 10129458.1 is used.

Furthermore, polyazoles are preferred which have been obtained by themethods described in German patent application No. 10117687.2.

The preferred polymers include polysulphone, in particular polysulphonewith aromatic and/or heteroaromatic groups in the main chain. Accordingto a particular aspect of the present invention preferred polysulphonesand polyethersulphones have a melt volume-flow rate MVR 300/21.6 of lessthan or equal to 40 cm³/10 min, in particular less than or equal to 30cm³/10 min with particular preference for less than or equal to 20cm³/10 min measured according to ISO 1133 In this case polysulphoneswith a Vicat Softening Temperature VST/A/50 of 180° C. to 230° C. arepreferred. In a further preferred embodiment of the present inventionthe mean molecular weight of the polysulphone is greater than 30,000g/mol.

The polysulphone-based polymers include in particular polymers havingrecurring units with concatenating sulphone groups of general formula A,B, C, D, E, F and/or G:

in which the radicals R independently of each other in the same way ordifferently represent an aromatic or heteroaromatic group, theseradicals having been explained in detail previously. These include inparticular 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4′-biphenyl,pyridine, chinolin, naphthalin and phenanthren.

The polysulphones preferred within the context of the present inventioninclude homo- and copolymers, for example statistical copolymers.Particularly preferred polysulphones contain recurring units of formulasH to N:

The polysulphones described above can be obtained commercially under thetrade names ®Victrex 200 P, ®Victrex 720 P, ®Ultrason E, ®Ultrason S,®Mindel, ®Radel A, ®Radel R, ®Victrex HTA, ®Astrel and ®Udel.

In addition, polyetherketone, polyetherketonketone,polyetheretherketone, polyetheretherketonketone and polyarylketone areparticularly preferred. These high performance polymers are inthemselves known and can be obtained commercially under the trade nameVictrex® PEEK™, ®Hostatec and ®Kadel.

The above mentioned polymers can be used individually or as a mixture(blend). In this case the particular preference is for blends containingpolyazole and/or polysulphone. Using blends allows the mechanicalcharacteristics to be improved and the material costs to be reduced.

The polymer film can also be further modified, for example bycross-linking as described in German patent application No. 10110752.8or in WO 00/44816. In a preferred embodiment the polymer film in a basicpolymer and at least one blending component that is used for expansionalso contains a cross-linking agent as described in patent applicationNo. 10140147.7.

It is also an advantage if the polymer film used for expansion is firsttreated as described in German patent application No. 10109829.4. Thisvariant is advantageous in increasing the expansion of the polymer film.

Instead of the polymer films produced using conventional methodspolyazole-containing polymer membranes as described in patentapplications Nos. 10117686.4, 10144815.5, 10117687.2 can also be used.Here these have the polyphosphoric acid and/or the phosphoric acidremoved and are used in step A). The inventive polymer membrane can alsohave further fillers and/or accessory agents.

To further improve the application engineering properties the membranecan also have further fillers, in particular proton-conducting fillers,and additional acids, added. The addition can, for example, take placein step A). Furthermore, these additives,- if they are in liquid form,can also be added after polymerisation in accordance with step B).

Non-restrictive examples of proton-conducting fillers are:

-   -   Sulphates such as: CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄,        NaHSO₄, KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,    -   Phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃,        UO₂PO₄.3H₂O, H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄,        LiH₂PO₄, NH₄H₂PO₄, CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄,        H₅Sb₅P₂O₂₀,    -   Polyacids such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O        (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆,        HNbO₃, HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄    -   Selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,        (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,    -   Oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃    -   Silicates such as zeolites, zeolites (NH₄+), layer silicates,        tectosilicates, H-hatrolits, H-mordenits, NH₄-analcines,        NH₄-sodalites, NH₄-gallates, H-montmorillonites    -   Acids such as HClO₄, SbF₅    -   Fillers such as carbides, in particular SiC, Si₃N₄, fibres, in        particular glass fibres, glass powders and/or polymer fibres,        preferably polyazole-based.

These additives can be contained in the proton-conducting polymermembrane in the normal quantities, but with the positive properties,such as high conductivity, long lifetime and high mechanical stabilityof the membrane not being too adversely affected by the addition ofexcessive quantities of additives. In general the membrane contains,after polymerisation in accordance with step B), a maximum of 80% byweight, preferably a maximum of 50% by weight, with particularpreference for a maximum of 20% by weight, of additives.

This membrane can also contain perfluorinated sulphonic acid additives(chiefly 0.1-20% by weight, preferably 0.2-15% by weight and mostpreferably 0.2-10% by weight). These additives result in performanceimprovements in the vicinity of the cathodes to increase the oxygensolubility and oxygen diffusion and to reduce the adsorption ofphosphoric acid and phosphate on platinum. (Electrolyte additives forphosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.; Olsen, C.; Berg,R. W.; Bjerrum, N. J. Chem. Dep. A, Tech. Univ. Denmark, Lyngby, Den. J.Electrochem. Soc. (1993), 140(4), 896-902 and Perfluorosulphonimide asan additive in phosphoric acid fuel cell. Razaq, M.; Razaq, A.; Yeager,E.; DesMarteau, Darryl D.; Singh, S. Case Cent. Electrochem. Sci., CaseWest. Reserve Univ., Cleveland, Ohio, USA. J. Electrochem. Soc. (1989),136(2), 385-90.)

Non-restrictive examples of persulphonated additives are:

-   -   trifluromethanesulphonic acid, potassium        trifluoromethanesulphonate, sodium trifluoromethanesulphonate,        lithium trifluoromethanesulphonate, ammonium        trifluoromethanesulphonate, potassium perfluorohexanesulphonate,        sodium perfluorohexanesulphonate, lithium        perfluorohexanesulphonate, ammonium perfluorohexanesulphonate,        perfluorohexanesulphonate acid, potassium        nonafluorobutanesulphonate, sodium nonafluorobutanesulphonate,        lithium nonafluorobutanesulphonate, ammonium        nonafluorobutanesulphonate, caesium nonafluorobutanesulphonate,        triethylammoniumperfluorohexasulphonate and        perfluorosulphonimide.

Vinyl-containing phosphoric acids are known in technical circles. Theseare compounds that have at least one carbon-carbon double bond and atleast one phosphonic acid group. Preferably the two carbon atoms whichform the carbon-carbon double bond have at least two, preferably threebonds with groups that lead to a low steric hindrance to the doublebond. These groups include, inter alia, hydrogen atoms and halogenatoms, in particular fluorine atoms. Within the context of the presentinvention the polyvinylphosphonic acid results from the polymerisationproduct which is obtained by polymerisation of the vinyl-containingphosphonic acid alone or with further monomers and/or cross-linkingagents.

The vinyl-containing phosphonic acid can contain one, two, three or morecarbon-carbon double bonds. Furthermore, the vinyl-containing phosphonicacid can contain one, two, three or more phosphonic acid groups.

In general the vinyl-containing phosphonic acid contains between 2 and20, preferably between 2 and 10, carbon atoms.

The vinyl-containing phosphonic acid used in step A) preferably involvescompounds of the formula

in which

-   -   R denotes a bond, a C1-C15-alkyl group, C1-C15-alkoxy group,        ethyleneoxy group or C5-C20-aryl or heteroaryl group, and the        abovementioned radicals for their parts can be substituted by        halogen, —OH, COOZ, —CN, NZ₂    -   Z independently of each other denotes hydrogen, a C1-C15-alkyl        group, C1-C15-alkoxy group, ethyleneoxy group or C5-C20-aryl or        heteroaryl group, and the abovementioned radicals for their        parts can be substituted by halogen, —OH, COOZ, —CN, NZ₂ and    -   x denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10    -   y denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10        and/or the formula        in which    -   R denotes a bond, a C1-C15-alkyl group, C1-C15-alkoxy group,        ethyleneoxy group or C5-C20-aryl or heteroaryl group, and the        abovementioned radicals for their parts can be substituted by        halogen, —OH, COOZ, —CN, NZ₂    -   Z independently of each other denotes hydrogen, a C1-C15-alkyl        group, C1-C15-alkoxy group, ethyleneoxy group or C5-C20-aryl or        heteroaryl group, and the abovementioned radicals for their        parts can be substituted by halogen, —OH, COOZ, —CN, NZ₂ and    -   x denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and/or        the formula        in which    -   A represents a group of formula COOR², CN, CONR² ₂, OR² and/or        R², in which R² denotes hydrogen, a C1-C15-alkyl group,        C1-C15-alkoxy group, ethyleneoxy group or C5-C20-aryl or        heteroaryl group, and the abovementioned radicals for their        parts can be substituted by halogen, —OH, COOZ, —CN, NZ₂,    -   R denotes a bond, a bivalent C1-C15-alkylene group, bivalent        C1-C15-alkyleneoxy group, for example an ethyleneoxy group or        bivalent C5-C20-aryl or heteroaryl group, and the abovementioned        radicals for their parts can be substituted by halogen, —OH,        COOZ, —CN, NZ₂,    -   Z independently of each other denotes hydrogen, a C1-C15-alkyl        group, C1-C15-alkoxy group, ethyleneoxy group or C5-C20-aryl or        heteroaryl group, and the abovementioned radicals for their        parts can be substituted by halogen, —OH, COOZ, —CN, NZ₂ and    -   x denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

The preferred vinyl-containing phosphonic acids include, inter alia,alkenes, which contain phosphonic acid groups, such as ethene phosphonicacid, propene phosphonic acid, butene phosphonic acid; acrylic acidand/or methacrylic acid compounds, which contain phosphonic acid groups,such as 2-phosphonomethyl-acrylic acid, 2-phosphonomethyl-methacrylicacid, 2-phosphonomethyl-acrylic amide and 2-phosphonomethyl-methacrylicacid amide.

Particular preference is for the use of commercially available vinylphosphonic acid (ethene phosphonic acid), such as that obtainable fromAldrich or Clariant G mbH. A preferred vinyl phosphonic acid has apurity of more than 70%, in particular 90% and with particularpreference for a purity of more than 97%.

The vinyl-containing phosphonic acids can also be used in the form ofderivates which can then be converted into the acid, with thisconversion to acid also being possible in the polymerised state. Thesederivatives include in particular the salts, esters, amides andhalogenides of the vinyl-containing phosphonic acids.

The expanded polymer film produced in step A) includes after expansionpreferably at least 10% by weight, in particular at least 50% by weightand with particular preference for at least 70% by weight, in relationto the total weight, vinyl-containing phosphonic acid. According to aparticular aspect of the present invention, the expanded polymer filmproduced in step A) includes a maximum of 60% by weight polymer film, inparticular a maximum of 50% by weight polymer film and with particularpreference for a maximum of 30% by weight polymer film, in relation tothe total weight. These variables can also be determined by the increasein weight brought about by the expansion.

The liquid used in step A) for expansion can also contain furtherorganic and/or inorganic solvents. The organic solvents include, inparticular, polar aprotic solvents, such as dimethylsulphoxide (DMSO),esters, such as ethyl acetate, and polar protic solvents, such asalcohols, ethanol, propanol, isopropanol and/or butanol. The organicsolvents include, in particular, water, phosphoric acid andpolyphosphoric acid.

These can have a positive influence on processability. In particular,the addition of the organic solvent can improve the expansion of themembrane. The content of vinyl-containing phosphonic acid in suchsolutions is at least 5% by weight, preferably at least 10% by weight,and with particular preference for between 10 and 97% by weight.

In a further embodiment of the invention the liquid containingvinyl-containing phosphonic acid also includes monomers that are capableof cross-linking. These are in particular compounds that have at least 2carbon-carbon double bonds. Preference is for diene, triene, tetraene,dimethylacrylate, trimethylacrylate, tetramethylacrylate, diacrylate,triacrylate and tetraacrylate.

Particular preference is for diene, triene, tetraene of the formula

dimethylacrylate, trimethylycrylate, tetramethylacrylate of the formula

and diacrylate, triacrylate, tetraacrylate of the formula

in which

-   -   R denotes a C1-C15-alkyl group, C5-C20-aryl or heteroaryl group,        NR′, —SO₂, PR′, or Si(R′)₂, and the abovementioned radicals can        be substituted,    -   R′ denotes independently of each other hydrogen, a C1-C15-alkyl        group, C1-C15-alkoxy group, C5-C20-aryl or heteroaryl group and    -   n is at least 2.

The substitutes for the abovementioned radical R preferably involvehalogen, hydroxyl, carboxy, carboxyl, carboxylester, nitrile, amine,ailyl and siloxane radicals.

Particularly preferred cross-linkers are allylmethacrylate,ethyleneglycoldimethacrylate, diethyleneglycoldimethacrylate,triethyleneglycoldimethacrylate, tetra- andpolyethyleneglycoldimethacrylate, 1,3-butanedioldimethacrylate,glycerinedimethacrylate, diurethanedimethacrylate,trimethylpropanetrimethacrylate, epoxyacrylate, such as Ebacryl,N′,N-methylenebisacrylamide, carbinol, butadiene, isoprene, chloroprene,divinylbenzene and/or bisphenol-A dimethylacrylate. These compounds arecommercially available, for example, from the Sartomer Company Exton,Pa. under the designations CN-120, CN104 and CN-980.

The use of cross-linking agents is optional, and these compounds canusually be used in the range 0.05 to 30% by weight, preferably 0.1 to20% by weight, and with particular preference for between 1 and 10% byweight, with reference to the vinyl-containing phosphonic acid.

The liquid containing the vinyl-containing phosphonic acid can be asolution and the liquid can also contain suspended and/or dispersedconstituents. The viscosity of the liquid containing vinyl-containingphosphonic acid can have a very wide range, and adjustment of theviscosity can be performed by addition of solvents or increasing thetemperature. The dynamic viscosity is preferably in the range 0.1 to10,000 mPa*s, in particular 0.2 to 2000 mPa*s, and these values can, forexample, be measured in accordance with DIN 53015.

The expansion of the film in step A) preferably takes place attemperatures in excess of 0° C., with particular preference for betweenambient temperature (20° C.) and 160° C. In principle the expansion canalso take place at low temperatures, but the time required for expansionincreases with a resultant drop in economic efficiency. At excessivelyhigh temperatures the film used for expansion can be damaged. Theexpansion time is dependent upon the temperature selected. The treatmenttime must be selected so that the desired expansion is achieved.

The polymerisation of the vinyl-containing phosphonic acid in step B)preferably takes place radically. The radical formation can take placethermally, chemically and/or electrochemically.

For example, a starter solution, which contains at least one substancecapable of radical formation, can be added to the liquid according tostep A). Furthermore, a starter solution can be applied to the expandedplanar formation. This can take place using in themselves known methods(such as spraying, immersion, etc.) which are part of the state of theart.

Suitable radical formers include azo compounds, peroxy compounds,persulphate compounds or azoamidine. Non-restrictive examples aredibenzoylperoxide, dicumol peroxide, cumol hydroperoxide,diisopropylperoxydicarbonate, bis(4-t-butylcyclohexyl)peroxydicarbonate, dipotassium persulphate, ammonium peroxydisulphate,2,2′-azobis(2-methylpropionitril) (AIBN), 2,2′-azobis-(isobutyric acidamine)hydrbchloride, benzpinacol, dibenzyl derivates,methylethylenketoneperoxide, 1,1-azobiscyclohexanecarbonitrile,methylethylketoneperoxide, acetylacetoneperoxide, dilaurylperoxide,didecanoylperoxide, tert.-butylper-2-ethylhexanoate, ketoneperoxide,methylisobutylketoneperoxide, cyclohexanoneperoxide, dibenzoylperoxide,tert.-butylperoxybenzoate, tert.-butyl peroxyisopropylcarbonate,2,5-bis(2-ethylhexanoyl-peroxy)-2,5-dimethylhexane,tert.-butylperoxy-2-ethylhexanoate, tert.-butyl peroxy-3,5,5-trimethylhexanoate, tert.-butyl peroxyisobutyrate, tert.-butylperoxyacetate,dicumylperoxide, 1,1 -bis(tert.-butylperoxy)cyclohexane,1,1-bis(tert.-butylperoxy)3,3,5-trimethylcyclohexane,cumylhydroperoxide, tert.-butylhydroperoxide,bis(4-tert.-butylcyclohexyl)peroxydicarbonate, and the radical formersavailable from DuPont under the ®Vazo name, such as ®Vazo V50 and ®VazoWS.

Radical formers can also be used that form radicals under irradiation.The preferred compounds include, inter alia, α,α-diethoxyacetophenone(DEAP, Upjon Corp), n-butyl benzoine ether ((®Trigonal-14, AKZO) and2,2-dimethoxy-2-phenylacetophenone (®Igacure 651) and1-benzoylcyclohexanol (®Igacure 184),bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide (®Irgacure 819) and1-[4-(2-hydroxyethoxy) phenyl]-2-hydroxy-2-phenylpropan-1-one (®Irgacure2959), which are each commercially available from Ciba Geigy Corp.

Normally between 0.0001 and 5% by weight, in particular between 0.01 and3% by weight (with reference to the vinyl-containing phosphonic acid) ofradical formers are added.The quantity of radical formers can be variedaccording to the desired degree of polymerisation.

The polymerisation can also take place under the effect of IR or NIR(IR=infrared, i.e. light with a wavelength of more than 700 nm; NIR=nearinfrared, i.e. light with a wavelength in the range approx. 700 to 2,000nm or an energy in the range approx. 0.6 to 1.75 eV).

The polymerisation can also take place under the effect of UV light witha wavelength of less than 400 nm. This polymerisation method is initself known and is, for example, described in Hans Joerg Elias,Makromolekulare Chemie, 5th Edition, Volume 1, pp. 492-511; D. R.Arnold, N. C. Baird, J. R. Bolton, J. C. D. Brand, P. W. M Jacobs, P. deMayo, W. R. Ware, Photochemistry—An Introduction, Academic Press, NewYork and M. K. Mishra, Radical Photopolymerization of Vinyl Monomers, J.Macromol. Sci.-Revs. Macromol. Chem. Phys. C22(1982-1983) 409.

The polymerisation can also be achieved through the effects of β-, γ-and/or electron radiation. According to a particular embodiment of thepresent invention a membrane is irradiated with a radiation dose in therange 1 to 300 kGy, preferably 3 to 200 kGy and with a quite particularpreference for 20 to 100 kGy.

The polymerisation of the vinyl-containing phosphonic acid in step B)preferably takes place at temperatures in excess of ambient temperature(20° C.) and below 200° C., in particular at temperatures of between 40°C. and 150° C., with particular preference for between 50° C. and 120°C. The polymerisation preferably takes place under normal pressure, butcan also take place under pressurisation. The polymerisation leads to ahardening of the expanded polymer film according to step A), and thishardening can be monitored using micro-hardness measurement. Theincrease in hardness caused by the polymerisation is preferably at least20%, in relation to the hardness of the polymer film expanded in stepA).

According to a particular embodiment of the present invention themembranes have a high mechanical stability. This variable is a result ofthe hardness of the membrane, which is determined using micro-hardnessmeasurement in accordance with DIN 50539. For this the membrane issuccessively stressed with a Vickers diamond for 20 seconds at up to aforce of 3 nM and the penetration depth is determined. Accordingly thehardness at ambient temperature is at least 0.01 N/mm², preferably atleast 0.1 N/mm² with quite particular preference for at least 1 N/mm²,without this being restrictive. Then the force is kept constant for 5 sat 3 mN and the creep is calculated from the penetration depth. Withpreferred membranes the creep C_(HU) 0.003/20/5 under these conditionsis less than 20%, preferably less than 10% and with a particularpreference for less than 5%. The modulus determined by means ofmicro-hardness measurement is at least 0.5 MPa, in particular at least 5MPa with quite particular preference for at least 10 MPa, without thisbeing restrictive.

Depending on the desired degree of polymerisation the planar formation,which is obtained by expansion of the polymer film and subsequentpolymerisation, is a self-supporting membrane. The degree ofpolymerisation is preferably at least 2, in particular at least 5, witha particular preference for at least 30 recurring units, in particularat least 50 recurring units with a quite particular preference for atleast 100 recurring units. This degree of polymerisation is determinedfrom the mean of the molecular weight M_(n) which can be determinedusing GPC methods. Because of the problems of isolating thepolyvinylphosphonic acid contained in the membrane without decomposingit, this value is determined by a test which is performed bypolymerisation of vinyl phosphonic acid without solvent and withoutaddition of polymers. In this case the proportion by weight of vinylphosophonic acid and radical starter compared with the ratios followingdissolution of the membrane is kept constant. The conversion that isachieved in a comparative polymerisation is preferably greater than orequal to 20%, in particular greater than or equal to 40% with aparticular preference for greater than or equal to 75%, with referenceto the vinyl-containing phosphonic acid used.

The inventive polymer membrane contains between 0.5 and 97% by weight ofpolymer and between 99.5 and 3% by weight polyvinylphosphonic acid. Theinventive polymer membrane preferably contains between 3 and 95% byweight of the polymer and between 97 and 5% by weightpolyvinylphosphonic acid, with particular preference for between 5 and90% by weight of the polymer and between 95 and 10% by weightpolyvinylphosphonic acid. In addition the inventive polymer membrane canalso contain further fillers and/or accessory agents.

Following the polymerisation in accordance with step C) the membrane canbe cross-linked thermally, photochemically, chemically and/orelectrochemically on the surface. This hardening of the membrane surfacefurther improves the properties of the membrane.

According to a particular aspect the membrane can be heated to atemperature of at least 150° C., preferably at least 200° C. with aparticular preference for at least 250° C. The thermal cross-linkingpreferably takes place in the presence of oxygen. The oxygenconcentration in this step of the process is normally in the range 5 to50% by volume, preferably 10 to 40% by volume, without this beingrestrictive.

The cross-linking can also take place under the effect of IR or NIR(IR=infrared, i.e. light with a wavelength of more than 700 nm; NIR=nearinfrared, i.e. light with a wavelength in the range approx. 700 to 2,000nm or an energy in the range approx. 0.6 to 1.75 eV) and/or UV-light.Another method is irradiation with β-, γ- and/or electron radiation. Theradiation dose in this case is preferably between 5 and 200 kGy, inparticular between 10 and 100 kGy. The irradiation can be performed inair or under an inert gas. This improves the usage properties of themembrane, in particular the durability.

Depending on the desired degree of cross-linking the duration of thecross-linking reaction can fall within a wide range. In general thereaction time is in the range from 1 second to 10 hours, preferably 1minute to 1 hour, without this being restrictive.

The inventive polymer membrane has improved material properties comparedwith the previously known doped polymer membranes. In particular,compared with known un-doped polymer membranes they already have anintrinsic conductivity. This is due in particular to the presence of apolymeric polyvinylphosphonic acid. The intrinsic conductivity of theinventive membrane at temperatures of 160° C. is generally at least0.001 S/cm, preferably at least 10 mS/cm, in particular at least 15mS/cm and with a particular preference for at least 20 mS/cm. Thesevalues are achieved without humidification.

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in potentiostatic mode and using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collection electrodes is 2 cm. The spectrum obtained isevaluated using a simple model comprising a parallel arrangement of anohmic resistor and a capacitor. The sample cross-section of thephosphoric acid-doped membrane is measured immediately prior to thesample being mounted. In order to measure the temperature-dependency themeasurement cell is heated in an oven to the desired temperature andadjusted by, means of a Pt-100 thermocouple in the immediate vicinity ofthe sample. Once the temperature has been reached the sample ismaintained at this temperature for 10 minutes before starting.

According to a particular embodiment the inventive membranes have aparticularly low methanol crossover. This variable can be expressed bythe crossover current density.

The crossover current density, during operation with 0.5 M methanolsolution and at 90° C. in a so-called liquid direct methanol fuel cell,is preferably less than 100 mA/cm², in particular less than 70 mA/cm²with a quite particular preference for less than 50 mA/cm² and an evengreater preference for less than 10 mA/cm². The crossover currentdensity, during operation with 2 M methanol solution and at 160° C. in aso-called gaseous fuel direct methanol fuel cell, is preferably lessthan 1 00 mA/cm², in particular less than 50 mA/cm² with a quiteparticular preference for less than 10 mA/cm².

In order to determine the crossover current density the carbon dioxidemixture that is released at the cathode is measured using a CO₂ sensor.The crossover current density is determined from the value of the CO₂quantity thereby obtained as described by P. Zelenay, S. C. Thomas, S.Gottesfeld in S. Gottesfeld, T. F. Fuller “Proton Conducting MembraneFuel Cells II” ECS Proc. Vol. 98-27pp. 300-308.

Possible areas of use for the inventive polymer membranes include, interalia, application in fuel cells, in electrolysis, in capacitors and inbattery systems. Because of the profile of their characteristics polymermembranes are preferably used in fuel cells.

The present invention also concerns a membrane-electrode assembly, whichhas at least one inventive polymer membrane. The membrane-electrodeassembly has a high operating efficiency, even with a low content ofcatalytically active substances, such as platinum ruthenium orpalladium. Gas diffusion layers with a catalytically active coating canbe used for this.

The gas diffusion layer generally demonstrates electron conductivity.Planar, electrically conducting and acid-resistant formations arenormally used for this. These include, for example, carbon fibre paper,graphitised carbon fibre paper, carbon fibre fabric, graphitised carbonfibre fabric and/or planar formations which are rendered conductive byaddition of carbon black.

The catalytically active layer contains a catalytically activesubstance. These include, inter alia, noble metals such as platinum,palladium, rhodium, iridium and/or ruthenium. These substances can alsobe used in the form of alloys with each other. Furthermore, thesesubstances can also be used in alloys with base metals such as Cr, Zr,Ni, Co and/or Ti. In addition, the oxides of the abovementioned noblemetals and/or base metals can also be used.

According to a particular aspect of the present invention thecatalytically active compounds are used in the form of particles whichpreferably have a size in the range 1 to 1,000 nm, in particular 10 to200 nm and preferably 20 to 100 nm.

The catalytically active particles, which the abovementioned substancescontain, can be used as metal powder, so-called black noble metal, inparticular platinum and/or platinum alloys. Such particles generallyhave a size in the range 5 nm to 200 nm, preferably in the range 10 nmto 100 nm.

Moreover, the metals can also be used on a base material. The basepreferably contains carbon which, in particular, can be used in the formof carbon black, graphite or graphitised carbon black. The metal contentof these supported particles, with reference to the total weight of theparticles, is generally in the range 1 to 80% by weight, preferably 5 to60% by weight with a particular preference for 10 to 50% by weight,without this being restrictive. The particle size of the base, inparticular the size of the carbon particles, is preferably in the range20 to 100 nm, in particular 30 to 60 nm. The size of the metal particlespresent on this is preferably in the range 1 to 20 nm, in particular 1to 10 nm with a particular preference for 2 to 6 nm.

The sizes of the various particles are means of the average weights andcan be determined by transmission electron microscopy.

The catalytically active particles defined above are generallycommercially available.

Furthermore the catalytically active layer can contain the normaladditives. These include, inter alia, fluoropolymers such aspolytetrafluoroethylene (PTFE) and surface-active substances.

The surface-active substances include in particular ionic surfactants,such as fatty acid salts, in particular sodium laurate, potassiumoleate; and alkyl sulphonic acids, alkyl sulphonic acid salts, inparticular sodium perfluorohexanesulphonate, lithiumperfluorohexanesulphonate, ammonium perfluorohexanesulphonate,perfluorohexanesulphonic acid, potassium nonafluorobutanesulphonate andnon-ionic surfactants, in particular ethoxylated fatty alcohols andpolyethylene glycol.

Particularly preferred additives are fluoropolymers, in particulartetrafluoroethylene polymers. According to a particular embodiment ofthe present invention the weight ratio between the fluoropolymer and thecatalyst material, comprising at least a noble metal and possibly one ormore base materials, is greater than 0.1, with this ratio preferablybeing in the range 0.2 to 0.6.

According to a particular embodiment of the present invention thecatalyst layer has a thickness in the range 1 to 1,000 μm, in particular5 to 500, preferably 10 to 300 μm. This value is an average, which canbe determined by measuring the layer thickness in the cross-section ofshots obtained using a raster electron microscope (REM).

According to a particular embodiment of the present invention the noblemetal content of the catalyst layer is 0.1 to 10.00 mg/cm², preferably0.3 to 6.0 mg/cm² with a particular preference for 0.3 to 3.0 mg/cm².These values can be determined by elementary analysis of a planarsample.

The production of a membrane-electrode assembly can, inter alia, takeplace by hot pressing. Here the combination of electrode, comprising gasdiffusion layers provided with catalytically active layers, and amembrane is heated to a temperature in the range 50° C. to 200° C. andpressed with a pressure of 0.1 to 5 MPa. In general a few seconds issufficient to bond the catalyst layer with the membrane. This time ispreferably in the range 1 second to 5 minutes, in particular 5 secondsto 1 minute.

The object of the present invention is likewise an inventiveproton-conducting polymer membrane coated with a catalyst layer.

Various methods can be used to apply the catalyst layer to the membrane.So, for example, a support can be used which is provided with a coatingcontaining a catalyst in order to provide the inventive membrane with acatalyst layer.

In this case the membrane can be provided with the catalyst layer on oneor both sides. If the membrane is provided with a catalyst layer on oneside only, the other side of the membrane must be pressed onto anelectrode that has a catalyst layer. If both sides of the membrane areprovided with a catalyst layer, the following methods can also be usedin combination in order to achieve an optimum result.

According to the invention, the catalyst layer can be applied using amethod in which a catalyst suspension is used. Furthermore powderscontaining the catalyst can also be used.

The catalyst suspension contains a catalytically active substance. Thesubstances have been listed above in connection with the catalyticallyactive layer.

Furthermore the catalyst suspension can contain normal additives. Theseinclude, inter alia, fluoropolymers such as polytetrafluoroethylene(PTFE), thickening agents, in particular water-soluble polymers such ascellulose derivatives, polyvinylalcohol, polyethyleneglycol, andsurface-active substances that have been defined above in connectionwith the catalytically active layer.

The surface-active substances include in particular ionic surfactants,such as fatty acid salts, in particular sodium laurate, potassiumoleate; and alkyl sulphonic acids, alkyl sulphonic acid salts, inparticular sodium perfluorohexanesulphonate, lithiumperfluorohexanesulphonate, ammonium perfluorohexanesulphonate,perfluorohexanesulphonic acid, potassium nonafluorobutanesulphonate andnon-ionic surfactants, in particular ethoxylated fatty alcohols andpolyethylene glycol.

Furthermore, the catalyst suspension can contain constituents that areliquid at ambient temperature. These include, inter alia, organicsolvents, which can be polar or nonpolar, phosphoric acid,polyphosphoric acid and/or water. The catalyst suspension preferablycontains 1 to 99% by weight, in particular 10 to 80% by weight liquidconstituents.

The polar, organic solvents include, in particular, alcohols such asethanol, propanol, isopropanol and/or butanol.

The organic nonpolar solvents include, inter alia, known thin filmdiluting agents, such as thin film diluting agent 8470 from DuPont,containing gum spirits of turpentine.

Particularly preferred additives are fluoropolymers, in particulartetrafluoroethylene polymers. According to a particular embodiment ofthe present invention the weight ratio between the fluoropolymer and thecatalyst material, comprising at least a noble metal and possibly one ormore base materials, is greater than 0.1, with this ratio preferablybeing in the range 0.2 to 0.6.

The catalyst suspension can be applied to the inventive membrane by theusual methods. Depending on the viscosity of the suspension, which canalso be in paste form, a number of methods are known by which thesuspension can be applied. Suitable methods are those for coating films,fabrics, textiles and/or papers, in particular spray methods andprinting methods such as template or silk-screen printing, ink-jetmethods, roller application, in particular screen rolling, slot dieapplication and doctoring. The particular method and the viscosity ofthe catalyst suspension are dependent upon the hardness of the membrane.

The viscosity can be affected by the proportion of solid matter content,in particular the content of catalytically active particles, and theproposition-of additives. The viscosity to be selected is dependent uponthe method of application of the catalyst suspension, the optimum valuesand determination of these being familiar to the person skilled in theart.

Depending on the hardness of the membrane an improvement in the bondingbetween the catalyst and the membrane can be achieved by heating and/orpressing.

According to a particular aspect of the present invention the catalystlayer is applied using a powder method. In this case a catalyst powderis used which can also contain additives which have been defined aboveby way of example.

In order to apply the catalyst powder spray and screen methods, interalia, can be used. With the spray method the powder mixture is sprayedvia a nozzle, for example a slot die, onto the membrane. Generally themembrane with the catalyst layer applied is then heated in order toimprove the bond between the catalyst and the membrane. Heating can, forexample, be performed using a hot roller. Such methods and devices forapplication of the powder are described, inter alia, in DE 195 09 748,DE 195 09 749 and DE 197 57 492.

With the screen method the catalyst powder is applied to the membraneusing a vibrating screen. A device for applying a catalyst powder to amembrane is described in WO 00/26982. Following the application of thecatalyst powder the bond between catalyst and membrane can be improvedby heating. In this case the membrane provided with at least a catalystlayer can be heated to a temperature in the range 50 to 200° C., inparticular 100 to 180° C.

Moreover, the catalyst layer can be applied using a method in which acoating containing a catalyst is applied to a carrier and then thecoating containing a catalyst on the carrier is transferred to theinventive membrane. Such a method is described, by way of example, in WO92/15121.

The carrier provided with a catalyst coating can, for example, beproduced by producing a catalyst suspension as described above. Thiscatalyst suspension is then applied to a carrier film, for example inpolytetrafluoroethylene. Following the application of the suspension thevolatile constituents are removed.

The transfer of the coating containing a catalyst can take place, interalia, by hot pressing. Here the combination comprising a catalyst layerand a membrane as well as a carrier film is heated to a temperature inthe range 50° C. to 200° C. and pressed with a pressure of 0.1 to 5 MPa.In general a few seconds is sufficient to bond the catalyst layer withthe membrane. This time is preferably in the range 1 second to 5minutes, in particular 5 seconds to 1 minute.

According to a particular embodiment of the present invention thecatalyst layer has a thickness in the range 1 to 1,000 μm, in particular5 to 500, preferably 10 to 300 μm. This value is an average, which canbe determined by measuring the layer thickness in the cross-section ofshots obtained using a raster electron microscope (REM).

According to a particular embodiment of the present invention themembrane provided with at least one catalyst layer contains 0.1 to 10.00mg/cm², preferably 0.3 to 6.0 mg/cm² with a particular preference for0.3 to 3.0 mg/cm². These values can be determined by elementary analysisof a planar sample.

Following the coating with a catalyst the membrane obtained can becross-linked thermally, photochemically, chemically and/orelectrochemically. This hardening of the membrane further improves theproperties-of the membrane. Here the membrane can be heated to atemperature of at least 150° C., preferably at least 200° C. with aparticular preference for at least 250° C. According to a particularembodiment the cross-linking preferably takes place in the presence ofoxygen. The acid concentration in this step of the process is normallyin the range 5 to 50% by volume, preferably 10 to 40% by volume, withoutthis being restrictive.

The cross-linking can also take place under the effect of IR or NIR(IR=infrared, i.e. light with a wavelength of more than 700 nm; NIR=nearinfrared, i.e. light with a wavelength in the range approx. 700 to 2,000nm or an energy in the range approx. 0.6 to 1.75 eV) and/or UV-light.Another method is irradiation with β-, γ- and/or electron radiation. Theradiation dose in this case is preferably between 5 and 200 kGy, inparticular between 10 and 100 kGy. The irradiation can be performed inair or under an inert gas. This improves the usage properties of themembrane, in particular the durability.

Depending on the desired degree of cross-linking the duration of thecross-linking reaction can fall within a wide range. In general thereaction time is in the range from 1 second to 10 hours, preferably 1minute to 1 hour, without this being restrictive.

The inventive polymer membrane with catalyst coating has improvedmaterial properties compared with the previously known doped polymermembranes. In particular, they perform better than known doped polymermembranes. This is due, in particular, to a better contact betweenmembrane and catalyst.

In order to produce a membrane-electrode assembly the inventive membranecan be bonded to a gas diffusion layer. If the membrane is provided witha catalyst layer on both sides, the gas diffusion layer must notcomprise any catalyst prior to pressing.

An inventive membrane-electrode assembly has a surprisingly high powerdensity. According to a particular embodiment preferredmembrane-electrode assemblies provide a current density of at least 0.1A/cm², preferably 0.2 A/cm² with a particular preference for 0.3 A/cm².This current density is measured during operation with hydrogen at theanode and air (approx. 20% by volume oxygen, approx. 80% by volumenitrogen) at the cathode under normal pressure (absolute 1,013 mbar,with open cell output) and 0.6V cell voltage. In this case particularlyhigh temperatures in the range 150-200° C., preferably 160-180° C., inparticular 170° C., can be used.

The abovementioned power densities can also be achieved at lowstoichiometry of the combustion gas on both sides. According to aparticular aspect of the present invention the stoichiometry is lessthan or equal to 2, preferably less than or equal to 1.5 with a quiteparticular preference for less than or equal to 1.2. According to aparticular embodiment of the present invention the catalyst layer has alow noble metal content. The noble metal content of a preferred catalystlayer, which an inventive membrane contains, is preferably a maximum of2 mg/cm², in particular a maximum of 1 mg/cm² with a quite particularpreference for a maximum of 0.5 mg/cm². According to a particular aspectof the present invention one side of a membrane has a higher metalcontent than the other side of the membrane. The metal content of oneside is preferably double the metal content of the other side.

In a further variant a catalytically active layer can be applied to theinventive membrane and bonded to this with a gas diffusion layer. Forthis a membrane is formed in accordance with steps A) and B) and thecatalyst is applied. In a variant the catalyst can be applied before ortogether with the starter solution. These formations are also an objectof the present invention.

Moreover, the formation of the membrane in accordance with steps A) andB) can also take place on a carrier or a carrier film which already hasthe catalyst. Following removal of the carrier or the carrier film, thecatalyst is located on the inventive membrane. These formations are alsoan object of the present invention.

Also an object of the present invention is a membrane-electrode assemblywhich contains at least an inventive polymer membrane if necessary incombination with another polymer membrane with a polyazole base or apolymer blend membrane.

Possible areas of use for the inventive polymer membranes include, interalia, use in fuel cells, in electrolysis, in capacitors and in batterysystems. Because of the profile of their characteristics polymermembranes are preferably used in fuel cells.

EXAMPLES 1 and 2

A film in high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is inserted in a solution comprising 10 parts by weight ofvinyl phosphonic acid (97%) available from Clariant and one part byweight of an aqueous solution containing 5 per cent by weight of2,2′-azo-bis-(isobutyric acid amidine)-dihydroxychloride. After variousinsertion times samples are taken and these are then treated in the ovenat 80° C. for 1 hour. The membrane obtained in this way has theconductivity at 160° C. determined by means of impedance spectroscopy.The mechanical properties (modulus of elasticity, hardness (HU) andcreep Cr) were determined after thermal treatment by micro-hardnessmeasurement. For this the membrane is successively stressed with aVickers diamond for 20 seconds at up to a force of 3 nM and thepenetration depth is determined. Then the force is kept constant for 5seconds at 3 mN and the creep from the penetration depth is calculated.The properties of these membranes are summarised in Table 1. TABLE 1Properties of PBI films following expansion with vinyl phosphonicacid-containing solution at ambient temperature Modulus InsertionConductivity @ of time 160° C. HU elasticity Cr [h] [mS/cm] [MPa] [MPa][%] Ex. 1 3 1.5 3 72.5 3.3 Ex. 2 22 10.7 1.7 37.6 4.3

EXAMPLES 3 to 5

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE101 10752.8. Then excess water is dabbed from the PBI film pre-treated-in this way using a paper cloth. This un-doped PBI film is then placedin a solution comprising 10 parts by weight vinyl phosphonic acid (97%)available from Clariant and one part by weight of an aqueous solutioncontaining 5 per cent by weight of 2,2′-azo-bis-(isobutyric acidamidine)-dihydroxychloride at 45°. Next, after various insertion times,the increase in weight, the increase in thickness and the increase inarea are determined. Then the membrane is treated in the oven at 80° C.for 1 hour. The membrane obtained in this way has the conductivity at160° C. determined by means of impedance spectroscopy. The properties ofthese membranes are summarised in Table 2. TABLE 2 Properties of washeddoped PBI films following expansion with vinyl phosphonicacid-containing solution at increased temperature In- Increase IncreaseConductivity sertion Insertion in Increase in @ time temp. thickness inarea weight 160° C. [h] [° C.] [%] [%] [%] [mS/cm] Ex. 3 1 45 118 51 1754.7 Ex. 4 4 45 133 110 525 16.4 Ex. 5 22 45 164 156 712 26.6

Elementary analyses were performed on the samples from examples 4 and 5,the results of which are presented in Table 2a. TABLE 2a wt % C wt % Hwt % O wt % N wt % P n(P)/n(N) Ex. 4 27.9 5.2 43 1.6 21.4 6.04 Ex. 526.6 5.15 42.5 1.7 23.3 6.19

The mechanical properties (modulus of elasticity, hardness (HU) andcreep Cr) were determined after thermal treatment by micro-hardnessmeasurement, and the results obtained are presented in Table 2b. TABLE2b Modulus of HU elasticity Cr [MPa] [MPa] [%] Ex. 4 0.42 8.4 2.1 Ex. 50.5 15.3 4.1

EXAMPLES 6 to 9

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE1 0110752.8. Then excess water is dabbed from the PBI film pre-treatedin this way using a paper cloth. This un-doped PBI film is then placedin a solution comprising 1 part by weight water and 10 parts by weightvinyl phosphonic acid (97%) available from Clariant at 60° C. for 2hours. The PBI film pre-expanded in this way is then placed for 24 hoursat ambient temperature in a solution comprising 10 parts by weight vinylphosphonic acid (97%) available from Clariant and one part by weight ofan aqueous solution containing 0.1-5 per cent by weight of2,2′-azo-bis-(isobutyric acid amidine)-dihydroxychloride, theconcentrations from the examples being presented in Table 3. Theincrease in thickness and increase in area are then determined. Then themembrane is treated in the oven at 80° C. for 1 hour and the increase inweight is determined. The membrane obtained in this way has theconductivity at 160° C. determined by means of impedance spectroscopy.The properties of these membranes are summarised in Table 3. TABLE 3Properties of washed doped PBI films following expansion with vinylphosphonic acid-containing solution and differing concentrations of thestarter 2,2′- azo-bis-(isobutyric acid amidine)-dihydroxychloride.Increase Conductivity Starter in Increase Increase @ concentrationthickness in area in weight 160° C. [%] [%] [%] [%] [mS/cm] Ex. 6 0.01145 143 589 13.9 Ex. 7 0.1 138 150 522 14.0 Ex. 8 1 162 144 574 14.4 Ex.9 5 142 129 503 12.5

The mechanical properties (modulus of elasticity, hardness (HU) andcreep Cr) were determined after thermal treatment by micro-hardnessmeasurement, and the data obtained are presented in Table 3a. TABLE 3aModulus of HU elasticity Cr [MPa] [MPa] [%] Ex. 6 0.5 10.4 3.1 Ex. 70.36 7.5 2.5 Ex. 8 0.42 8 2.7 Ex. 9 0.7 13.7 2.9

EXAMPLES 10 to 14

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE101 10752.8. Then excess water is dabbed from the PBI film pre-treatedin this way using a paper cloth. This un-doped PBI film is then placedin a solution comprising 1 part by weight water and 10 parts by weightvinyl phosphonic acid (97%) available from Clariant at 80° C. forbetween 1.5 and 2.5 hours. The PBI film pre-expanded in this way is thenplaced for 24 hours at ambient temperature in a solution comprising 10parts by weight vinyl phosphonic acid (97%) available from Clariant andone part by weight of an aqueous solution containing 0.1 per cent byweight 2,2′-azo-bis-(isobutyric acid amidine)-dihydroxychloride. Theincrease in thickness and increase in area are then determined. Then themembrane is treated in the oven at 80° C. for 1 hour and the increase inweight is determined. The membrane obtained in this way has theconductivity at 160° C. determined by means of impedance spectroscopy.The properties of these membranes are summarised in Table 4. Themechanical properties of this membrane with a weight increase of between500 and 600 wt % vary between 0.4 and 0.7 MPa for the HU hardness (HU),7 and 14 MPa for the modulus of elasticity and 2 and 4% for the creep.TABLE 4 Properties of washed doped PBI films following expansion withvinyl phosphonic acid-containing solution and the same production methodIncrease in Increase Conductivity @ thickness in area Increase in 160°C. [%] [%] weight [%] [mS/cm] Ex. 10 142 150 600 — Ex. 11 143 113 50315.3 Ex. 12 142 143 560 18.6 Ex. 13 148 124 545 16.7 Ex. 14 149 135 56019.4

EXAMPLE 15

A film in polybenzimidazole, which has been produced from a PBI-DMAcsolution in accordance with DE 10052237.8 and by addition of across-linker and a blend component in accordance with DE 10140147.7, isinitially washed at 45° for 30 minutes as described in DE10110752.8.Then excess water is dabbed from the PBI film pre-treated in this wayusing a paper cloth. This un-doped PBI film is then placed in a solutioncomprising 1 part by weight water and 10 parts by weight vinylphosphonic acid (97%) available from Clariant at 70° C. for 3 hours. ThePBI film pre-expanded in this way is then placed for 24 hours at ambienttemperature in a solution comprising 10 parts by weight vinyl phosphonicacid (97%) available from Clariant and one part by weight of an aqueoussolution containing 0.1 per cent by weight 2,2′-azo-bis-(isobutyric acidamidine)-dihydroxychloride. The increase in thickness and increase inarea are then determined. Then the membrane is treated in the oven at80° C. for 1 hour and the increase in weight is determined. The membraneobtained in this way has the conductivity at 160° C. determined by meansof impedance spectroscopy. The properties of these membranes aresummarised in Table 5. TABLE 5 Properties of a high-strength PBImembrane following expansion with vinyl phosphonic acid-containingsolution Increase in Increase Increase Conductivity @ thickness in areain weight 160° C. [%] [%] [%] [mS/cm] Ex. 15 94 170 307 17

The mechanical properties of such a membrane were determined bymicro-hardness measurement. The membrane has a hardness (HU) of 1.2N/mm², a modulus of elasticity, Y, of 28 MPa and a creep, Cr, of 9.5%.

EXAMPLES 16 to 19

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE10110752.8. Then excess water is dabbed from the PBI film pre-treatedin this way using a paper cloth. This un-doped PBI film is then placedin a solution comprising 1 part by weight water and 10 parts by weightvinyl phosphonic acid (97%) available from Clariant at 70° C. for 2hours. The PBI film pre-expanded in this way is then placed for 24 hoursat ambient temperature in a solution comprising 10 parts by weight vinylphosphonic acid (97%) available from Clariant and 0.1-2%N,N′-methylenebisacrylamide and one part by weight of an aqueoussolution containing 0.1 per cent by weight 2,2′-azo-bis-(isobutyric acidamidine)-dihydroxychloride. The increase in thickness and increase inarea are then determined. Then the membrane is treated in the oven at130° C. for 3 hours and the increase in weight is determined. Themembrane obtained in this way has the conductivity at 160° C. determinedby means of impedance spectroscopy. The properties of these membranesare summarised in Table 6. TABLE 6 Properties of a high-strength PBImembrane following expansion with vinyl phosphonic acid-containingsolution containing various cross-linker concentrations IncreaseConductivity Cross-linker in Increase Increase in @ concentrationthickness in area weight 160° C. [wt %] [%] [%] [%] [mS/cm] Ex. 16 0.1145 131 472 16.6 Ex. 17 0.5 151 137 489 15.2 Ex. 18 1 145 131 481 16.3Ex. 19 2 128 119 463 15.1

The mechanical properties of these samples were determined bymicro-hardness measurement. The results are summarised in Table 6a.TABLE 6a Modulus Cross-linker of Sample concentration elasticity HU CrNo. [wt %] [MPa] [MPa] [%] Ex. 16 0.1 9.1 0.43 2.4 Ex. 17 0.5 11.2 0.572.2 Ex. 18 1 9.1 0.48 2.6 Ex. 19 2 12.2 0.58 2.6

EXAMPLES 20 to 23

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE10110752.8. Then excess water is dabbed from the PBI film pre-treatedin this way using a paper cloth. This un-doped PBI film is then isplaced in a solution comprising 1 part by weight water and 10 parts byweight vinyl phosphonic acid (97%) available from Clariant at 70° C. for2 hours. The increase in thickness and increase in area are thendetermined. The membrane is then treated by electron irradiation andwith a radiation dose of 33-200 kGy. The membrane obtained in this wayhas the conductivity at 160° C. determined by means of impedancespectroscopy. The properties of these membranes are summarised in Table7b. TABLE 7 Properties of vinyl phosphonic acid containing PBI membranesprior to electron irradiation Increase in Increase in Example thickness[%] area [%] 20 139 130 21 139 130 22 145 130 23 138 150

The mechanical properties of these samples were determined bymicro-hardness measurement. The results are summarised in Table 7a.TABLE 7a Irradiation Conductivity @ Modulus of dose 160° C. elasticityHU Cr Example [kGy] [mS/cm] [MPa] [MPa] [%] 20 33 9.5 370 9 5.5 21 663.7 70 3.9 6.1 22 99 2 1880 39.2 7.1 23 200 1 139 8.3 7.3

In order to determine the content of acid that can be washed away theirradiated membranes are in a first stage placed in water at ambienttemperature, agitated for 10 minutes, and the acid released isdetermined following removal of the membrane by titration from theconsumption with 0.1 molar caustic soda up to the second titrationpoint. In a second stage, the membrane sample is treated in a beakerwith boiling water for 30 minutes. The acid thereby released is againdetermined by means of titration from the consumption with 0.1 molarcaustic soda up until the second titration point. In a third stage, themembrane pre-treated in this way is again treated for 30 minutes withboiling water and the acid thereby released is again determined by meansof titration. The results are obtained are presented in Table 7b. Ifthis procedure is performed with a non-irradiated membrane from examples10 to 14, then the consumption of 0.1 molar caustic soda up until thesecond end point in the first stage is 28-36 ml, in the second stageless than 2 ml and in the third stage 0.2 ml. TABLE 7b V(0.1 M V(0.1 MNaOH) NaOH) V(0.1 M NaOH) Irradiation after stage 1 after stage 2 afterExample dose [kGy] [ml] [ml] stage 3 [ml] 20 33 16.1 0.04 0 21 66 15.00.09 0.04 22 99 11.2 0.3 0.08 23 200 6.8 0.5 0.3

EXAMPLE 24

A membrane-electrode assembly is produced by pressing a membrane fromexample 11 and 2 electrodes with a Pt content of 1 mg/cm² at the anodeand 2 mg/cm² at the cathode. A temperature of 140° C., a pressing timeof 30 s and a pressure of 4 N/mm² are selected for the pressing. An MEAproduced in this way with an active surface area of 10 cm² is operatedwithout humidification in a single cell at 160° C. After 16 hours ofoperation a hydrogen flow of 5.7 I/h and an air flow of 22.5 I/h thefollowing performance data are produced at an absolute pressure p_(a) of1 bar and 2 bar. TABLE 9 Performance data for an MEA in accordance withexample 23 p_(a) I [bar] [A/cm²] 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11.1 1 U [mV] 855 731 682 644 612 582 552 522 487 451 408 351 2 U [mV]890 770 726 695 668 644 620 596 570 543 514 480

EXAMPLE 25

A membrane-electrode assembly is produced by pressing a membraneirradiated with 66 kGy from example 21 and 2 electrodes with a Pt/Rucontent of 1.5 mg/cm² at the anode and 4 mg/cm² Pt black at the cathode.A temperature of 120° C., a pressing time of 30 s and a force of 5 kNare selected for the pressing. An MEA produced in this way with anactive surface area of 30 cm² is initially maintained at 90° C. with 0.5molar methane solution and a flow of 20 ml/min for 16 hours atequilibrium rest potential. The methanol crossover is measured by meansof a CO₂ sensor at the cathode output. The methanol crossover is 70mA/cm² compared with 100 mA/cm² for an identical cell containing aNafion 117 membrane. The cell resistance is 355 mOhm*cm² compared with144 mOhm*cm² for an identical cell containing a Nafion 117 membrane. Theequilibrium rest potential is 780 mV compared with 730 mV for anidentical cell containing a Nafion 117 membrane. Then the followingperformance data are obtained with this direct-methanol-cell. TABLEPerformance data for a direct-methanol-cell at 90° with 0.5 M MeOH.p_(a) I [bar] [A/cm²] 0 0.02 0.06 0.08 0.1 0.2 0.3 0.4 3 U [V] 0.78 0.650.595 0.56 0.53 0.45 0.39 0.28

EXAMPLE 26

A membrane-electrode assembly is produced by pressing a membrane fromexample 11 and 2 electrodes with a Pt content of 1 mg/cm² at the anodeand 2 mg/cm² at the cathode. A temperature of 140° C., a pressing timeof 30 s and a pressure of 4 N/mm² are selected for the pressing. Theactive surface area is 30 cm². An MEA produced in this way is thentreated with electron irradiation and an irradiation dose of 99 kGy. AnMEA produced in this way with an active surface area of 30 cm² isinitially maintained at 90° C. with 0.5 molar methane solution and aflow of 20 ml/min for 16 hours at equilibrium rest potential. Themethanol crossover is measured by means of a CO₂ sensor at the cathodeoutput. The methanol crossover is 9 mA/cm². The cell resistance is 944mOhm*cm². The equilibrium rest potential is 750 mV. After 1 hour'soperation at 0.2 A/cm² with a 0.5 M MeOH solution at 90° C. the cell isoperated with a 10 M MeOH solution. In operation with 10 M MeOH solutionat 90° C. and a flow of 20 ml/min the methanol crossover is 90 mA/cm²and the equilibrium rest potential is 610 mA/cm². After 1 hour ofoperation with 10 M MeOH the cell is again operated with 0.5 M MeOHsolution and identical results are obtained for equilibrium restpotential, cell resistance and methanol crossover as at the start of themeasurement.

EXAMPLE 27

A film in a high molecular weight polybenzimidazole, which has beenproduced from a PBI-DMAc solution in accordance with DE 10052237.8 andby selection of suitable polymer granulate in accordance with DE10129458.1, is initially washed at 45° for 30 minutes as described inDE10110752.8. Then excess water is dabbed from the PBI film pre-treatedin this way using a paper cloth. The un-doped PBI film is then placed ina solution containing 50 g vinyl phosphonic acid (97%) available fromClariant, 4.463 g bisphenol-A epoxy diacrylate (CN-120 from SartomerInc.) and 2 g 1-hydroxycyclohexyl phenyl ketone (Irgacure 184 fromCiba-Geigy) for 2 hours at 70° C. in a darkened chamber. The membraneexpanded in this way is placed between 2 paper cloths and rolled 10times with a cylinder weighing 250 g.

The film is then placed between 2 transparent films of orientedpolypropylene and excess air is removed by repeated rolling as describedabove. This laminate is then transferred to a chamber where each side isirradiated for 1 minute with a 300 W mercury arc lamp of the H3T7 typefrom General Electric and this process is repeated once. Thepolypropylene film is carefully removed from the membrane. This processis made easier by gentle heating using a hot-air oven. A typical weightincrease following this treatment is 500 wt %.

1-12. (canceled)
 13. A proton-conducting electrolyte membrane obtainedby a method consisting of the steps: a) expanding a polymer film with aliquid that contains a vinyl-containing phosphonic acid, and b)polymerizing the vinyl-containing phosphonic acid present in the liquidof step a), characterized in that the intrinsic conductivity of theinventive membrane at temperatures of 160° C. is at least 0.001 S/cm.14. The membrane of claim 13, characterized in that the film used instep a) has an expansion of at least 3% in the liquid containingvinyl-containing phosphonic acid.
 15. The membrane of claim 13,characterized in that the polymers used in step a) are high-temperaturestable polymers which contain at least one nitrogen, oxygen, or sulphuratom in one or more recurring units.
 16. The membrane of claim 13,characterized in that the liquid containing the vinyl-containingphosphonic acid contains compounds of the formula

in which R denotes a bond, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂ Z independently of each other denotes hydrogen, a C1-C15alkyl group, C1-C15 alkoxy group, ethyleneoxy group, C5-C20 aryl orheteroaryl group, and the abovementioned radicals are optionallysubstituted by halogen, —OH, COOZ, —CN, NZ₂ and x denotes a whole number1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 y denotes a whole number 1, 2, 3, 4, 5,6, 7, 8, 9 or 10 or the formula

in which R denotes a bond, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, Z independently of each other denotes hydrogen, a C1-C15alkyl group, C1-C15 alkoxy group, ethyleneoxy group, C5-C20 aryl orheteroaryl group, and the abovementioned radicals are optionallysubstituted by halogen, —OH, COOZ, —CN, NZ₂, and x denotes a wholenumber 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, or the formula

in which A represents a group of formula COOR², CN, CONR² ₂, OR², or R²,in which R² denotes hydrogen, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, or C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, R denotes a bond, a bivalent C1-C15 alkylene group,bivalent C1-C15 alkyleneoxy group, and the abovementioned radicals areoptionally substituted by halogen, —OH, COOZ, —CN, NZ₂, Z independentlyof each other denotes hydrogen, a C1-C15 alkyl group, C1-C15 alkoxygroup, ethyleneoxy group, or C5-C20-aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, and x denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9or
 10. 17. The membrane of claim 13, characterized in that the liquidcontaining the vinyl-containing phosphonic acid contains monomers thatare capable of cross-linking.
 18. The membrane of claim 13,characterized in that the liquid containing the vinyl-containingphosphonic acid contains at least one substance that is capable ofradical formation.
 19. The membrane of claim 13, characterized in thatthe polymerization of step c) takes place by irradiation with IR light,NIR light, UV light, β-radiation, γ-radiation, or electron radiation.20. The membrane of claim 13, characterized in that the membrane has anintrinsic conductivity of at least 0.001 S/cm.
 21. The membrane of claim13, characterized in that the membrane contains between 0.5 and 97% byweight of polymer and between 99.5 and 3% by weight polyvinylphosphonicacid.
 22. The membrane of claim 13, characterized in that the membranehas a layer containing a catalytically active component.
 23. Amembrane-electrode assembly containing at least an electrode and atleast one proton-conducting electrolyte membrane obtained by a methodconsisting of the steps: a) expanding a polymer film with a liquid thatcontains a vinyl-containing phosphonic acid, and b) polymerizing thevinyl-containing phosphonic acid present in the liquid of step a),characterized in that the intrinsic conductivity of the inventivemembrane at temperatures of 160° C. is at least 0.001 S/cm.
 24. Theassembly of claim 23, characterized in that the film used in step a) hasan expansion of at least 3% in the liquid containing vinyl-containingphosphonic acid.
 25. The assembly of claim 23, characterized in that thepolymers used in step a) are high-temperature stable polymers whichcontain at least one nitrogen, oxygen, or sulphur atom in one or morerecurring units.
 26. The assembly of claim 23, characterized in that theliquid containing the vinyl-containing phosphonic acid containscompounds of the formula

in which R denotes a bond, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂ Z independently of each other denotes hydrogen, a C1-C15alkyl group, C1-C15 alkoxy group, ethyleneoxy group, C5-C20 aryl orheteroaryl group, and the abovementioned radicals are optionallysubstituted by halogen, —OH, COOZ, —CN, NZ₂ and x denotes a whole number1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 y denotes a whole number 1, 2, 3, 4, 5,6, 7, 8, 9 or 10 or the formula

in which R denotes a bond, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, Z independently of each other denotes hydrogen, a C1-C15alkyl group, C1-C15 alkoxy group, ethyleneoxy group, C5-C20 aryl orheteroaryl group, and the abovementioned radicals are optionallysubstituted by halogen, —OH, COOZ, —CN, NZ₂, and x denotes a wholenumber 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, or the formula

in which A represents a group of formula COOR², CN, CONR² ₂, OR², or R²,in which R² denotes hydrogen, a C1-C15 alkyl group, C1-C15 alkoxy group,ethyleneoxy group, or C5-C20 aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, R denotes a bond, a bivalent C1-C15 alkylene group,bivalent C1-C15 alkyleneoxy group, and the abovementioned radicals areoptionally substituted by halogen, —OH, COOZ, —CN, NZ₂, Z independentlyof each other denotes hydrogen, a C1-C15 alkyl group, C1-C15 alkoxygroup, ethyleneoxy group, or C5-C20-aryl or heteroaryl group, and theabovementioned radicals are optionally substituted by halogen, —OH,COOZ, —CN, NZ₂, and x denotes a whole number 1, 2, 3, 4, 5, 6, 7, 8, 9or
 10. 27. The assembly of claim 23, characterized in that the liquidcontaining the vinyl-containing phosphonic acid contains monomers thatare capable of cross-linking.
 28. The assembly of claim 23,characterized in that the liquid containing the vinyl-containingphosphonic acid contains at least one substance that is capable ofradical formation.
 29. The assembly of claim 23, characterized in thatthe membrane has an intrinsic conductivity of at least 0.001 S/cm. 30.The assembly of claim 23, characterized in that the membrane containsbetween 0.5 and 97% by weight of polymer and between 99.5 and 3% byweight polyvinylphosphonic acid.
 31. The assembly of claim 23,characterized in that the membrane has a layer containing acatalytically active component.
 32. A fuel cell containing: one or moreproton-conducting electrolyte membranes obtained by a method consistingof the steps: a) expanding a polymer film with a liquid that contains avinyl-containing phosphonic acid, and b) polymerizing thevinyl-containing phosphonic acid present in the liquid of step a),characterized in that the intrinsic conductivity of the inventivemembrane at temperatures of 160° C. is at least 0.001 S/cm, or one ormore of the proton-conducting electrolyte membranes.